SOME CHARACTERISTICS OF THE AIR 



11 



For instance, an empty bottle may be inverted and its mouth 

 placed under water, and the bottle warmed by holding it be- 

 tween the hands (fig. 11) or by using a flame. The air will 

 soon be warmed, and if it expands, bubbles of air will over- 

 flow from the mouth of the bottle and rise to the surface of 

 the water. What other ways can you devise to test the 

 notion that air expands when heated? 



9. Thermometers. Air expands when 

 heated and contracts when cooled. It is 

 possible to arrange apparatus in such a 

 way that this expansion maybe observed 

 and measured. An instrument by means 

 of which such measurements could be 

 made would give us some idea of the 

 degree of heat. It would, in fact, be 

 an air thermometer. Such thermometers 

 have been constructed and used, but 

 only for special purposes. For ordinary 

 purposes much more satisfactory ther- 

 mometers are made from other sub- 

 stances. 



Thermometers such as are in common 

 use depend upon the expansion and con- 

 traction of a liquid. The liquid most 

 commonly used is mercury, though 

 alcohol which has been colored red or 

 blue is sometimes used. Each thermom- 

 eter is furnished with a scale of degrees by means of which the 

 temperature may be read. There are two sorts of scales in use 

 in this country (fig. 12). The one in most common use is called 

 the Fahrenheit scale, after the man who designed it. On this 

 scale the freezing point of water is 32 and the boiling point 

 is 212. The centigrade scale, which is the one used in all 

 scientific work the world over, places the zero mark at the 

 freezing point of water and the 100 mark at the boiling point 



FIG. 11. Expansion of air 



When heated by the hands 

 the air in the flask expands 

 and bubbles of air appear 

 at the open end of the tube 

 under the water 



